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Aluminate glasses are important materials from a fundamental structural point of view because Al is the only network former. They present also a technological interest because of their good IR transmission and ultralow optical losses. Aluminum in glasses of the system MO–Al2O3 (M = Ca, Sr, Ba) can have different coordination numbers, essentially 4 and 6, as a function of the MO/Al2O3 ratio. Using Raman spectroscopy and high field 27Al NMR spectroscopy, we have determined the structure of aluminum network as a function of MO/Al2O3 ratio with M corresponds to different alkaline–earth cations. Al is essentially in four-fold coordination with different amounts for Al2O3 between 50 and 75% but varies between Q2 and Q4 species as a function of MO/Al2O3 ratio where Q is tetrahedral species and n the number of bridging oxygen.

In this paper, the influence of carbon dioxide on the combustion of aluminum droplets is investigated. Millimeter-sized droplets were heated and ignited by a laser in an aerodynamic levitation system in several CO2 containing atmospheres (H2O/CO2, H2O/CO2/N2) with a large range of compositions (wet – xH2O

The influence of nitrogen on the aluminum droplet combustion under forced convection conditions has been studied. An aerodynamic levitation technique of millimetric size liquid droplets heated with a CO2 laser has been adopted to characterize the combustion of aluminum droplets and, in particular, to observe the surface phenomena. The determination of the burning rate and of the droplet temperature in several atmospheres (H2O/O2, H2O/Ar, H2O/N2, and air) has shown that they depend only on the nature and concentration of the oxidizers (O2 and H2O); a comparison of experiments in nitrogen and in argon containing mixtures demonstrated that N2 did not influence the gas phase combustion. However, for nitrogen containing atmospheres we observed the formation of solid aluminum nitride (AlN) at the droplet surface after a latency time depending on the nitrogen pressure. AlN first interacts with the oxide cap producing an aluminum oxynitride, then completely covers the droplet, and finally prevents combustion. The existence of a latency time varying with the nitrogen pressure suggests that the AlN formation is controlled by heterogeneous kinetics. The phenomenon of oxide cap regression during combustion was also observed in all gases, and it is attributed to a chemical decomposition process of alumina by aluminum forming gaseous AlxOy species. Therefore, nitrogen effects are significant at the droplet surface rather than in the gas phase, and it is suggested that N2 is probably one of the main species causing the manifestation of unsteady processes during aluminum droplet burning.

New values of densities ρ and surface tensions σ of liquid aluminum obtained in the range 1600 to 2360 K by contactless techniques in neutral gases are reported. Conditions for oxygen-free aluminum are fulfilled which allow determination of the surface tension of aluminum. Extrapolation to the melting point, T m = 933 K, confirms the value of σ (T = 933K) = 1.05 N ⋅ m−1.

The influence of grain-boundaries on uranium and oxygen diffusion in polycrystalline UO2 has been investigated. Our results show that between 1498°C and 1697°C, in H2 atmosphere, uranium diffusion in UO2 grain-boundaries is about five orders of magnitude greater than uranium volume diffusion, in the same experimental conditions. Between 605°C and 750°C, in H2/N2/H2O atmosphere, the oxygen diffusion coefficients measured in polycrystalline and single crystalline UO2 are similar, and correspond to the volume diffusion.

The behavior of ionic conducting oxides is discussed. It is shown that if such an oxide is placed between short-circuited reversible electrodes exposed to different chemical potentials, the oxygen ions are moving essentially under the influence of a gradient of electrical potential.

The density of liquid alumina drops maintained in levitation with an aerodynamic device and heated with CO2 lasers is determined by analysis of high-speed video digital images between 2000 and 3100 K in various gases. It is shown that consistent results can be achieved for the lighter drops (m

Oxygen self-diAusion measurements determined below and above the glass transition temperature in a cordierite glass are reported. The diAusion experiments were performed by the solid‐gas isotope exchange method using 18 Oa s a tracer in H2‐H2 18 O and Ar/ 18 O2 atmospheres. The 18 O diAusion profiles were established by secondary ion mass spectrometry (SIMS) after appropriate diAusion annealing. The oxygen diAusion coeAcients changed at the glass transition temperature as expected. The atmosphere aAected the oxygen diAusivity in agreement with other studies. The diAusion data were useful for comparisons with diAusion controlled process in the same glass. There is a diAerence between the activation enthalpies for crystal growth, viscous flow, and oxygen self-diAusion. Hence, bond-breaking and molecular reorientation required for crystallization and the atomic transport mechanism involved in viscous flow are not correlated in any obvious way to oxygen diAusion in cordierite glass. ” 1998 Elsevier Science B.V. All rights reserved.

The measurement of the surface tension of liquid silicon has a long history with many results but no general agreement between them. Two values at the melting temperature are cited in reviews (749 and 827 mN/m [N. Eustathopoulos, E. Ricci, B. Drevet, Note Technique DEM No. 97/58, CEA, 1997]) but there are few arguments to determine the correct one. In the present study, new data for the surface tension obtained with the analysis of characteristic frequencies of a levitating drop are presented. The effect of oxygen and nitrogen are also considered. These data are compared with former data obtained with contactless techniques. The most recent surface tension values obtained with drop weights ranging on two orders of magnitude and environments of different natures (argon, hydrogen and vacuum) show excellent agreement (within a 1.5% margin) at temperatures between 1350 K and 2400 K. The comparison of these data to others obtained with different techniques, reveal a good agreement, except those obtained with the sessile drop technique on some supports like BN, SiO2 and MgO. However, these special cases may be connected with the reactivity of silicon with these supports.

International audience; The structure and properties of glasses and melts in the MgO-Al2O3-SiO2 (MAS) and CaO-MgO-Al2O3-SiO2 (CMAS) systems play an important role in Earth and material sciences. Aluminum has a crucial influence in these systems, and its environment is still questioned. In this paper, we present new results using Raman spectroscopy and 27Al nuclear magnetic resonance on MAS and CMAS glasses. We propose an Al/Si tetrahedral distribution in the glass network in different Qn species for silicon and essentially in Q4 and VAl for aluminum. For the CMAS glasses, an increase of VAl and VIAl is clearly visible as a function of the increase of Mg/Ca ratio in the (Ca,Mg)3Al2Si3O12 (garnet) and (Ca,Mg)AlSi2O8 (anorthite) glass compositions. In the MAS system, the proportion of VAl and VIAl increases with decreasing SiO2 and, similarly with calcium aluminosilicate glasses, the maximum of VAl is located in the center of the ternary system.

Unidirectional solidification experiments were carried out at the eutectic composition of the systems NiO-ZrO{sub 2} and NiO-Y{sub 2}O{sub 3}, yielding materials made of regularly aligned lamellar structures. Eutectic samples were submitted to a chemical reduction treatment at 1075{degrees}C under CO/CO{sub 2} mixtures corresponding to {ital P}{sub O{sub 2}} values for which only NiO should be reduced to the metal. The rapid formation of nickel between the zirconia or yttria platelets and the morphology of the nickel layer resulting from reduction establish that the reduction kinetics were controlled predominantly by the diffusion of CO to the Ni/NiO interface through the porous Ni product. Experimental data of the variation of reduction depth of NiO as a function of time are qualitatively interpreted using a simple model of interdiffusion of CO in N{sub 2}/CO/CO{sub 2} mixtures. This model also explains the observation of an influence of the dilution of CO on the reduction depth.

The combustion of aluminum droplets in wet (3 mol% H 2 O) and dry CO 2 is studied in order to identify the influence of the two atmospheres on the surface processes. Millimeter-sized samples are maintained contactless in an aerodynamic levitation system and are heated continuously during burning by a laser. Ignition and combustion of the aluminum droplet are observed with a high-speed camera, the Al surface temperature is measured by an optical pyrometer, and unburnt residues are analyzed by X-ray diffraction. The determination of the burning rates and of the droplet temperatures reveals no differences between wet and dry CO 2 ( β = 1.28 ± 0.05 mm 2 / s , T = 2600 ± 50 K ), which shows that the gas-phase combustion regime is not affected by the presence of water vapor. However, the oxide cap, initially formed by the oxide coating breakdown at ignition, is progressively removed in wet CO 2 , whereas it is unvarying in dry CO 2 . Comparison between Al burning in CO 2 /H 2 and in CO 2 /(Ar or He) demonstrates that the oxide cap regression in a wet atmosphere is related to a chemical effect of hydrogen produced in the flame, and then diffusing and reacting at the droplet surface. It is suggested that the adsorption mechanism of H 2 on the Al surface may slow down the contribution of adsorbed oxygen-containing species (CO) to the oxide cap, which would consequently promote its decomposition (removal). Furthermore, the carbon dissolution process is observed in wet and dry CO 2 . When the carbon concentration reaches the saturation limit in the burning Al droplet ( x C = 0.23 at T = 2600 K ), the excess of carbon is ejected at the surface and forms a solid coating. In the absence of the oxide cap (wet CO 2 ), the refractory carbon coating prevents strong surface oxidation, and the combustion definitely stops. In the presence of the oxide cap (dry CO 2 ), the carbon coating reacts and produces an oxycarbide phase which is melted by the laser heating; a new burning regime occurs mainly controlled by direct surface reactions, and leading to the slow oxidation of the droplet and the expulsion of dissolved carbon into CO. Finally, a qualitative model of the combustion of aluminum in CO 2 atmospheres is proposed.

An infrared spectrometer (spectral range 2–6 μm), coupled with a 9 μm pyrometer (Christiansen’s wavelength), has been developed to collect time-resolved measurements (32 spectra/s) of the spectral emissivity of alumina droplets (d≈ 3 mm) freely cooling in an aerodynamic levitation system from the liquid-to-solid phases [2800 → 1500 K]. The temperature and nature of the gaseous atmosphere surrounding the droplet (oxidizing/neutral/reducing) are two important parameters affecting the spectral emissivity in the semi-transparent range. Observations are discussed in the framework of the thermal activation and of the chemical interactions of alumina with the environment.

Aerodynamic levitation allows contactless diagnostics of very high-temperature liquid properties. Liquids can be metals or inorganic compounds, and the levitation gas may have any composition. Temperature limitation is dependent on the heating power and radiative properties of the samples, with 3000°C possible for oxides. Surface-tension measurement is based on previous theories, which relate it to specific vibrations of the drop. Rotation and precession of liquid drops are specific aspects of aerodynamic levitation, and their effects are quantified here. Experimental examples for nickel and alumina are also shown and compared with results from previous experiments, when available.

Measurements of the thermophysical and structural properties of liquid materials at high temperature have undergone considerable development in the past few years. Following improvements in electromagnetic levitation, aerodynamic levitation associated with laser heating has shown promise for assessing properties of different molten materials (metals, oxides, and semiconductors), preserving sample purity over a wide range of temperatures and under different gas environments. The density, surface tension and viscosity are measured with a high-speed video camera and an image analysis system. Results on nickel and alumina show that small droplets can be considered in the first approximation to be under microgravity conditions. Using a non-invasive contactless technique recently developed to measure electrical conductivity, results have been extended to variety of materials ranging from liquid metals and liquid semiconductors to ionically conducting materials. The advantage of this technique is the feasibility of monitoring changes in transport occurring during phase transitions and in deeply undercooled states.